Immersion lithography offers an attractive alternative to 157-nm and EUV lithography. Due to the tremendous obstacles associated with these exotic technologies, immersion lithography has become the last technique for the incremental extension of dioptric/catadioptric optical lithography, if not the last technique for photon lithography per se.
The basic idea of immersion lithography is to fill up the air space between the last element of the imaging lens and the surface of the photoresist on the wafer, as shown in Fig. 7.1. With a higher-index medium, light of higher spatial frequencies can be coupled to the resist, improving the resolution. Liquid coupling also minimizes the reflection loss at the two affected interfaces. The classical interpretation of the improvement of resolution is that NA ≡ n · sinθ is increased with the change of refractive index n in the coupling medium. But, how does one visualize it?